As logic integrated circuits (ICs) have migrated to lower working voltages in the search for lower power consumption and higher operating frequencies, and as overall system sizes have continued to decrease, power supply designs with smaller size and higher efficiency are in demand. In an effort to improve efficiencies and increase power densities, synchronous rectification has become necessary for these type of applications. Synchronous rectification refers to using active devices such as the MOSFET as a replacement for Schottky diodes as rectifier elements in circuits to reduce conduction power losses in the secondary rectifiers. Recently, self-driven synchronous schemes have been widely adopted in the industry as the desired method for driving the synchronous rectifiers in DC/DC modules for output voltages of 5 volts and below. Self-driven synchronous schemes provide a simple, cost effective and reliable method of implementing synchronous rectification.
Most of these schemes are designed to be used with a very particular set of topologies commonly known as "D, 1-D" (complementary driven) type topologies. See Cobos, J. A., et al., "Several alternatives for low output voltage on board converters", IEEE APEC 98 Proceedings, at pp. 163-169. See also U.S. Pat. No. 5,590,032 issued on Dec. 31, 1996 to Bowman et al. for a Self-synchronized Drive Circuit for a Synchronous Rectifier in a Clamped-Mode Power Converter, and U.S. Pat. No. 5,274,543 issued on Dec. 28, 1993 to Loftus entitled Zero-voltage Switching Power Converter with Lossless Synchronous Rectifier Gate Drive. In these types of converters, the gate of the devices is referenced to ground, and the power transformer signal in the secondary winding has the correct shape and timing to directly drive the synchronous rectifiers with minimum effort. Furthermore, the rectifier is configured to insure the synchronous rectifier gate signals do not float relative to secondary ground and are easy to drive. FIG. 1 shows an example of this family of converters, with an active clamp forward circuit 10 and self-driven synchronous rectification provided by synchronous rectification circuitry 12 comprising two synchronous rectifiers SQ1 and SQ2 coupled between the secondary winding of the transformer 18 and the output, V.sub.out. As shown in FIG. 2, the transformer signal 20 for these types of converters has a square shape with two very recognizable intervals, each corresponding to the "on" time of one of the synchronous rectifiers SQ1 and SQ2.
In topologies such as the hard-switched half-bridge (HB), the full-bridge (FB) rectifiers, and the push-pull topologies and non-"D, 1-D" type topologies (e.g. clamp forward with passive reset), the transformer voltage has a recognizable zero voltage interval, making it undesirable to implement self-driven synchronous rectification. As a result, it is necessary to use an external drive circuit with these circuit topologies. Changing the placement of the synchronous rectifiers relative to the transformer to simplify the driving scheme may result in a floating transformer winding with respect to ground, which generally increases common mode current between the primary and secondary circuits, causing increased EMI noise. Rectifier circuits employing synchronous rectification generally are reconfigured away from the EMI-preferred configuration.
What is needed in the art is a circuit and method for providing synchronous rectification for the secondary side of a transformer that is suitable for use with a wide range of circuit topologies and has low EMI noise.